1. Field of the Invention
This invention relates to a semiconductor device having a fixed memory, comprising a semiconductor body having a surface-adjoining surface region of a first conductivity type in which a number of juxtaposed, parallel strip-shaped surface zones of the second conductivity type are provided, the surface being covered with an insulating layer on which a number of juxtaposed, parallel strip-shaped conductor tracks are provided which cross the strip-shaped surface zones, in which depending on the information stored in the memory, the conductor tracks at the area of some of the crossings, can be connected to the strip-shaped zones by a rectifying junction through windows in the insulating layer.
2. Description of the Prior Art
The invention relates in addition to a method of manufacturing such a device.
Memories of the above-described kind are generally known and are usually referred to in literature by the abbreviation ROM (read only memory). The strip-shaped surface zones situated in the body and the conductor tracks provided on the insulating layer constitute a cross-bar system of address lines and read lines for selecting and reading the memories at the crossings. The information (logic 1 and 0 or conversely) corresponds to the presence or absence of a diode connection at the area of the crossings between the address lines.
In general, rapid diodes should be used in said types of memories, that is to say, diodes having short switching times and/or recovery times. Such a semiconductor device is known from the article "A Bipolar 16K ROM Utilizing Schottky Diode Cells" by J. F. Gunn et al., IEEE International Solid-State Circuits Conference 1977, pp. 118/119. In the semiconductor memory described in said article the conductor tracks (word lines) are formed by strips of a metal which, through windows in the insulating layer, form a rectifying Schottky junction with the strip-shaped surface zones in the semiconductor body. The strip-shaped surface zones, termed bit lines, consist of three sub-zones, namely a comparatively low-doped central zone which forms the Schottky junctions with the metal tracks, and two comparatively highly doped zones on either side of the central zone. This structure is comparatively compact and has the additional advantage that, as described in the publication, standard bipolar technologies may be used for the manufacture thereof. As a result of this, further circuit elements, such as transistors, necessary for selecting and/or reading the memory and possibly also for the further processing of the stored information, can be integrated in the same semiconductor body, as is usually desired.
In this known device, the highly doped subzones which are situated on either side of the central sub-zone and which primarily determine the resistances of the bit-lines and are separated laterally from the Schottky junction, occupy much space, at least as compared with the (imaginary) case in which said highly doped zones would coincide with the central zone. However, such a configuration is not possible because usually good and reliable Schottky junctions can be formed only on comparatively high-ohmic semiconductor material. Principally only ohmic (that is to say non-rectifying) contacts can be obtained on low-ohmic material.